In healing bone fractures it is desirable to compress the fractures so that the fractured surfaces are pressed against one another. In the prior art, bone screws have been used to draw the fractured surfaces together and thereby optimize the healing process.
A number of prior art bone screws have been constructed in a fashion resembling wood screws. For example, some prior art bone screws include a threaded distal portion and a head with a relatively long unthreaded shank disposed between the head and the distal portion. A drill is used to create a bore through the fracture and the screw is threaded into the remote bone fragment with the head of the screw compressing the near fragment tightly against the remote bone fragment.
Other bone screws are threaded along the length thereof, thus requiring a first drill bit to create a bore in both bone fragments extending across the fracture and a second bit to drill a larger bore in the near bone fragment so that the screw threads do not engage the near bone fragment. Thereafter, the screw is tightened in the same manner as described above in connection with the screw having an unthreaded shank, thereby compressing the fragments together.
The operation of two prior art headed lag screws is illustrated in FIGS. 8A-10D. The operation of a lag screw A.sub.1 with a head B.sub.1 and a shank C.sub.1 is shown in FIGS. 8A-D. Shank C.sub.1 of screw A.sub.1 includes threads D.sub.1 at the distal end and an unthreaded region E.sub.1 proximal to head B.sub.1. The pitch of threads D.sub.1 is constant. FIG. 8A shows screw A.sub.1 partially engaged in a bore F.sub.1 in a near bone fragment G.sub.1. The diameter of bore F.sub.1 is less than the diameter of threads D.sub.1 and therefore the threads engage the walls of the bore as the screw is twisted in. FIG. 8B shows screw A.sub.1 as it starts threading into a bore H.sub.1 in a remote bone fragment I.sub.1. At this point threads D.sub.1 are engaged in both bores and moving forward at the same speed in both fragments so no compression between the fragments is achieved. Head B.sub.1 has reached the top of fragment G.sub.1 in FIG. 8C, as indicated schematically by the radiating "force" lines. Since threads D.sub.1 are no longer engaged in fragment G.sub.1, screw A.sub.1 rotates freely in the fragment without being drawn forward therein. Subsequent rotation of screw A.sub.1 draws fragment I.sub.1 further up the screw. Because head B.sub.1 prevents fragment G.sub.1 from moving further up screw A.sub.1, fragment I.sub.1 is drawn up against fragment G.sub.1 and compression between the fragments is achieved as shown in FIG. 8D, with the head pulling down on the near fragment and the threads pulling up on the remote fragment.
The importance of the unthreaded region of screw A.sub.1 is illustrated in FIGS. 9A-d. A lag screw A.sub.2 including a head B.sub.2 and a shank C.sub.2 is shown partially engaged in a bore F.sub.2 in a near fragment G.sub.2 in FIG. 9A. Shank C.sub.2 includes threads D.sub.2 running the entire length with no unthreaded region such as E.sub.1 on screw A.sub.1. Rotating screw A.sub.2 causes it to be drawn through fragment G.sub.2 and pass into a bore H.sub.2 in a remote fragment I.sub.2, as shown in FIG. 9B. Further rotation of screw A.sub.2 brings head B.sub.2 down against the upper surface of fragment G.sub.2. See FIG. 9C. At this point, threads D.sub.2 are still engaged in bore F.sub.2 of fragment G.sub.2 and the interaction of the head on the surface of fragment G.sub.2 impedes the further rotation of screw A.sub.2. To have additional rotation, head B.sub.2 would have to be drawn down into fragment G.sub.2 or the portion of threads D.sub.2 in fragment G.sub.2 would have to strip out. Therefore a fully threaded screw, such as screw A.sub.2, would not be preferred for use in the fragment and bore configuration of FIGS. 9A-D.
The proper bore configuration for using screw A.sub.2 is illustrated in FIGS. 10A-D. As shown in FIG. 10A, bore F.sub.2 in fragment G.sub.2 is enlarged to allow threads D.sub.2 of screw A.sub.2 to pass freely through the bore. Screw A.sub.2 therefore slips into bore F.sub.2 until it reaches fragment I.sub.2. At that point, threads D.sub.2 engage the walls of bore H.sub.2 and draw screw A.sub.2 down into fragment I.sub.2. See FIGS. 10B-C. When head B.sub.2 reaches the upper surface of fragment G.sub.2, further rotation causes fragment I.sub.2 to be drawn up into contact with fragment G.sub.2 as shown in FIGS. 10C-D. No binding occurs between head B.sub.2 and threads D.sub.2 in the near fragment because of the large bore in fragment G.sub.2, and the screw functions as intended to draw the two fragments together.
FIGS. 11A-12D illustrate the effect of substituting headless screws in the place of lag screws A.sub.1 and A.sub.2. FIG. 11A, in particular, shows a headless screw A.sub.3 partially installed in a bore F.sub.3 in a near fragment G.sub.3. Screw A.sub.3 includes threads D.sub.3 extending along its entire length. The pitch of threads D.sub.3 is constant. FIG. 11B shows screw A.sub.3 extending through fragment G.sub.3 and just entering a bore H.sub.3 in a remote fragment I.sub.3. FIG. 11C shows screw A.sub.3 advanced further into fragment I.sub.3. It should be noted that, since the pitch of threads D.sub.3 is constant, screw A.sub.3 moves forward in fragments G.sub.3 and I.sub.3 by the same amount with each rotation. As shown in FIG. 11D, screw A.sub.3 will pass through both fragments without altering their relative spacing or compressing them together. Thus, a headless screw such as screw A.sub.3 will not work to draw the fragments together in the same way as lag screws A.sub.1 and A.sub.2.
A variation of screw A.sub.3 is shown at A.sub.4 in FIG. 12A. Screw A.sub.4 includes threads D.sub.4 of constant pitch extending along its entire length and differs from screw A.sub.3 in that it tapers from a smaller outside diameter at the leading end to a larger outside diameter at the trailing end. Screw A.sub.4 is shown because it incorporates tapering, which is one of the features of the present invention, however, it is unknown whether such a screw is found in the prior art. Screw A.sub.4 is shown partially installed in a bore F.sub.4 in a near fragment G.sub.4 in FIG. 12A. As screw A.sub.4 is rotated, it moves through fragment G.sub.4 and into a bore H.sub.4 in a remote fragment I.sub.4, as shown in FIG. 12B. Subsequent rotation simply carries screw A.sub.4 further into and through fragment I.sub.4 without any effect on the spacing between the fragments. See FIGS. 12C-D. With a constant pitch thread, such as found on thread D.sub.4, the taper does not facilitate compression. Taper may, however, make a screw easier to start in a small pilot hole or even without a pilot hole. The threaded portion of many wood screws follows this general format, tapering to a sharp point, to allow installation without a pilot hole.
It can be seen from the above discussion that a headless screw of constant pitch does not achieve the desired compressive effect between the two fragments as will a lag screw with a head. It is, however, possible to draw two fragments together with a headless screw if it has varying pitch. FIG. 13A shows a headless screw A.sub.5 with threads D.sub.5 formed along its entire length. Such a screw is shown in U.S. Pat. No. 146,023 to Russell. The pitch of threads D.sub.5 varies from a maximum at the leading end to a minimum at the trailing end. It is expected that such a screw moves forward upon rotation in a fragment according to the approximate average pitch of the threads engaged in the fragment. Screw A.sub.5 is shown in FIG. 13A with the leading threads engaged in a bore F.sub.5 in a near fragment G.sub.5. Rotation of screw A.sub.5 causes it to move forward into and through fragment G.sub.5 and into a bore H.sub.5 in a remote fragment I.sub.5, as shown in FIG. 13B. Additional rotation after the leading threads engage fragment I.sub.5 causes the two fragments to be drawn together. See FIGS. 13C-D. This is because the average pitch of the threads in fragment I.sub.5 is greater than the average pitch of threads in fragment G.sub.5. Since the screw moves forward in each fragment with each 360.degree. rotation by an amount roughly equal to the average pitch of the threads in that fragment, each rotation will move the screw forward further in fragment I.sub.5 than in fragment G.sub.5. This effect will gradually draw the fragments together as the screw moves forward. Depending on the initial spacing between the fragments, they can make contact either before or after the trailing end of the screw has entered fragment G.sub.5. It should be noted that screw A.sub.5, in contrast to constant pitch screws such as screws A.sub.1 and A.sub.2, can be used to separate fragments G.sub.5 and I.sub.5 by simply reversing the rotation.
One drawback of a screw such as shown in Russell is the stripping or reaming of the female threads created in the bore by the leading threads as the trailing threads follow. Because the pitch changes along the length of the screw, no thread exactly follows the thread directly in front of it. Rather, each thread tends to cut its own new path which only partially overlaps the path of the thread ahead of it. Thus, the trailing threads tend to ream out the female threads in the bore made by the leading threads. This effect reduces the grip of the trailing threads and therefore the overall compressive force available to urge the fragments together.
FIG. 14A shows a headless screw A.sub.6, such as disclosed in U.S. Pat. No. 4,175,555 to Herbert, that offers one solution to the problem of reaming of threads. As noted in the Herbert patent, bone screws having heads suffer from several disadvantages including concentrated loads beneath the screw head and the protrusion of the screw head itself after the screw is installed. Several other shortcomings of the standard type of bone screw are detailed in the Herbert patent.
Screw A.sub.6, as per Herbert, includes a shank C.sub.6 with leading threads J.sub.6 at the leading end, trailing threads K.sub.6 at the trailing end and an unthreaded region E.sub.6 separating the leading and trailing threads. Threads J.sub.6 and K.sub.6 each have fixed pitch, but leading threads J.sub.6 have a larger pitch and smaller outside diameter than trailing threads K.sub.6. FIG. 14A shows leading threads J.sub.6 of screw A.sub.6 installed in a bore F.sub.6 of a near fragment G.sub.6. It should be noted that threads J.sub.6 do not engage the walls of bore F.sub.6, the bore having been bored large enough to allow leading threads J.sub.6 to pass freely. As the screw moves forward, the leading threads engage a bore H.sub.6 in a remote fragment I.sub.6. See FIG. 14B. The diameter of bore H.sub.6 is adapted so that leading threads J.sub.6 engage the walls. Meanwhile, at the trailing end of the screw, trailing threads K.sub.6 start to engage the walls of bore F.sub.6, which has been bored to an appropriate diameter therefor.
As soon as trailing threads K.sub.6 are engaged in bore F.sub.6 and leading threads J.sub.6 are engaged in bore H.sub.6, the two fragments start drawing together. See FIG. 14C. Further rotation of screw A.sub.6 completes the process of moving the two fragments together as shown in FIG. 14D. Screw A.sub.6 operates on the same general principle as screw A.sub.5, except that the average pitch of the threads in the remote and near fragments is simply the pitch of the leading and trailing threads, respectively. For instance, if the pitch of the leading threads is 0.2 inches and the pitch of the trailing threads is 0.1 inches, each rotation of screw A.sub.6 will move it 0.2 inches further into fragment H.sub.6, but only 0.1 inches further into fragment I.sub.6, thus moving the fragments 0.1 inches closer together.
The Herbert screw overcomes at least one of the drawbacks of the Russell screw, the reaming of female threads by subsequent threads on the screw, but at the same time suffers from a number of other disadvantages. In the Herbert screw, the leading threads have a smaller diameter than the trailing threads. This is necessary to permit the leading threads to pass through the relatively large bore in the near bone fragment and engage the smaller bore in the remote bone fragment. The larger trailing threads then engage the larger bore in the near bone fragment. As a result of this arrangement, any stripping of the threads cut into the bones during installation of the screw occurs in the remote bone. If the stripping occurred in the bore in the near bone fragment, a screw having a head thereon could still be used to compress the fracture even though the near bore was stripped; however, when stripping occurs in the bore in the remote bone, a standard screw with the head thereon cannot be used and another bore must be drilled.
Further, the Herbert screw must be correctly positioned, i.e., it is imperative that the fracture intersect the unthreaded central portion of the Herbert bone screw when the same is installed. Thus, the Herbert screw is not suitable for treating fractures that are very near the surface of the bone where the hole is to be drilled. In addition, because the Herbert screw is not threaded entirely along the length thereof, the purchase obtained by the screw in the bone is not as good as with a screw threaded along the entire length. Also, two bores of different sizes must be drilled to install the Herbert screw rather than a single bore.
Yet another problem with the Herbert screw is the stripping that can occur if additional tightening occurs after the screw has drawn the bone fragments together. While the bone fragments are being drawn together, trailing threads K.sub.6 all follow a single path through the near fragment. Similarly, leading threads J.sub.6 all follow a single path through the remote fragment. When, however, the bone fragments make contact the two sets of threads can no longer move independently. Further rotation of the Herbert screw after contact between the fragments can cause the leading threads to strip out as they attempt to move forward through the distal bone fragment faster than the trailing threads will allow. See The Herbert Bone Screw and Its Applications in Foot Surgery, The Journal of Foot and Ankle Surgery, No. 33, Vol. 4., 1994, pages 346-354 at page 346, which reports on a study that found compression of 10 kg. after only two complete turns of the trailing threads engaged in the near bone fragment. Each subsequent revolution lead to a decrease in compressive force. Thus, care must be taken not to over-tighten the Herbert screw.
In addition to drawing two bone fragments together to repair fractures, it is sometimes desirable to draw together two bones for fusing the same together in connection with arthrodesis of the interphalangeal joints. This procedure is sometimes indicated with symptoms of pain or instability in the finger joints. The purpose is to immobilize and draw together adjacent bones across a joint to cause them to fuse together thereby preventing further movement at the joint.
In one prior art procedure for immobilizing the distal interphalangeal joint (DIP), axial bores are drilled in the articular surfaces of the distal and proximal phalanges. The bore in the distal bone is sufficiently large to receive without threading a screw which is inserted therein via an incision in the tip of the finger. The screw threadably engages the bore in the proximal bone and when the screw head is tightened against the distal end of the distal bone, the two bones are compressed together. After several weeks, the bones fuse together. A second procedure to remove the screw must be performed because the head of the screw will cause discomfort in the finger pad if the screw is not removed.
This procedure is undesirable because it requires two separate surgeries. Katzman, et al., Use of a Herbert Screw for Interphalangeal Joint Arthrodesis, Clinical Orthopedics and Related Research, No. 296 pages 127-132 November 1993), describes use of the screw disclosed in the Herbert patent in procedures for interphalangeal joint arthrodesis.
Many of the above-discussed disadvantages associated with using a Herbert screw to compress a fracture are also present when the Herbert screw is used for interphalangeal joint arthrodesis.
It would be desirable to provide a headless bone screw which overcomes the disadvantages associated with the Herbert bone screw, as well as other prior art bone screws.